Blizzard have often changed art previously - sometimes with a fanfare, other times with barely a mention. They will likely continue to do so.
This is probably the most drastic of changes to a card art I've seen since they have also replaced the card with a whole new character - I'm not sure why the necessity, since there are other "Heroes" who are also cards, but Blizzard does what they do. Without any functionality changes to the card, while your annoyance is probably understandable, there's not much that cant be done. It's not really worthy of any sort of reimbursement (discretional as that is in any case).
I don't think you can argue that this is a nerf. At most the change is neutral. "After" is not strictly worse wording, and is situationally better. The summoned 2/1s are also demons, which gives them tribal synergy.
That said, since a big part of card games is the art, and golden cards specifically exist to put a premium cost on enhanced artwork, I don't think a full dust refund for changed art is an unreasonable request at all. Regardless of whether the card is good or bad, if a player crafts it because they like the art or featured character, it would at least be a nice courtesy for Blizzard to offer the same dust refund in the rare case that it is changed.
Now, does that argument holds any water regarding the game's ToS? Probably not. Even giving full refunds for nerfed cards is probably a courtesy they aren't obligated to do, since I'm sure their ToS specifies you don't own the cards and they can be changed without notice. So I doubt you will have much luck pursuing a refund.
Indeed, one can argue multiple examples for each side being better. My statement that Whenever is objectively better than After comes from the fact that Whenever has priority status, unlike After, and that is a mechanical advantage which is reflected on the timing of execution. Obviously, sometimes After will be better, but on an objective standpoint, an effect with priority status is better.
It's not for nothing that going first in Hearthstone is a big deal, priority status is an important aspect of a game that have all mechanics working based on timing.
Edit: Also, I'm not arguing anything from a legal standpoint, we are all renting access to our account without recourse in case we are locked out, that's not even a question.
Whenever to after is basicly a nerf, you can't play Crystal Weaver or Defender of Argus.
The new Token would not get the Buffs from the Battlecries. :/
Did they get those buffs to begin with though? Are we saying that Illidan's effect was that he summoned minions before the card was finished playing? Because "whenever" isn't really possible, there's always an order in which things happen because they can't happen simultaneously in hearthstone. If it was before, what happened if you played a minion with only one slot on the board - did Illidan summon a 2-1 and you couldn't play the card, or did he not summon one? Either way, the change is obviously a good thing if that's how he worked before, because that sounds inconsistent with internal logic of the game like hell.